Direct conversion detectors are promising for medical imaging applications, like SPECT, CT and X-ray imaging. One of the most promising direct conversion materials available at this moment is an alloy of cadmium, zinc and tellurium (CZT). If CZT will make it to a commercial detection system depends on performance, price and reliability. The good performance of CZT is known and has been demonstrated. However, cost of CZT detectors is high and good reliability on longer term has not been demonstrated yet.
According to a standard methodology, there are provided well defined indium contacts on a cadmium tellurium or a cadmium zinc tellurium made by evaporation, wherein indium serves as well as anode material and adheres well to the cadmium tellurium. An electrical contact to the cadmium tellurium or a cadmium zinc tellurium is then made by conductive adhesive printed on a printed circuit board. However, this may lead to an unreliable contact between the conductive adhesive and the indium surface, so that reliability problems may occur due to ingress of humidity and subsequent oxidation or hydroxylation of the indium surface. This may lead to degraded functional performance, for example caused by higher contact resistances.
Electrical contacts to a cadmium tellurium (CdTe) component, in particular to a cadmium zinc tellurium component, which are used in, for example, X-ray detectors for medical diagnostics, are nowadays formed on an indium layer or bond pad because of the stable resistive contact. The electrical contact to the indium bond pad usually is formed with a conductive adhesive joint. The alternative of soldering results often in a rapid dissolution of the indium layer. Takahashi et al, ‘High Resolution CdTe Detector and Applications to Imaging Devices’, IEEE Transactions on Nuclear Science, 48, Pages 287-291, 2001, describes a combination of soft metal, gold and indium as a stud to prevent possible stress on the device. In order to obtain a good connection between the bond pad on the read out board and the pixel electrode on the CdTe wafer, a needle shaped stud consisting of two stages of gold studs is prepared on the bump pad. The studs are made from a gold stud bonder and a thin layer of indium is printed on the top of the stud to improve connectivity. The CdTe wafer and the fan out board are then pressed together.
However, such an arrangement may lead to a poorly defined anode structure. A pressure contact is difficult to control for large detector surfaces, so that a cadmium tellurium component is easily damaged by applying pressure. Further, it is difficult to control the amount of indium placed on top of the gold stud, leading to a questionable reliability and industrialization.
Various metals are used as contact electrodes for CT or CZT detectors, including platinum, gold and indium. Detectors with indium contacts connected to printed circuit boards using isotropic conductive adhesives (ICA) suffer from degradation. The presently manufactured CZT detectors contain indium contacts (both cathode and anode side). Packaging related problems with indium are the low melting temperature, the reactivity towards air, moist and other metals and the mobility. Change to other electrode materials is not trivial and is not expected in the next years.
Several of the problems can be overcome by selecting proper (mild) processes and materials, and functional detectors have been made by using low temperature curing carbon filled adhesives to connect the CZT to the PCB. However, it was observed that the electrical resistance of such contacts gradually increase in time, which shows that on longer term the reliability of such a system is not guaranteed.